Recently, photonic crystals have been drawing attentions as a new optical device. A photonic crystal is an optical functional material having a periodic distribution of refractive index, which provides a band structure with respect to the energy of photons. One of its particular features is that it has an energy region (called the photonic bandgap) that does not allow the propagation of light.
An example of the application fields of the photonic crystal is the optical communication. Recent optical communications use the wavelength division multiplexing (WDM) in place of a conventional method called the time division multiplexing (TDM). The wavelength division multiplexing is a communication method in which plural wavelengths of light, each carrying a different signal, propagate through a single transmission line. This method has drastically increased the amount of information that can be transmitted per unit of time.
In wavelength division multiplexing, plural wavelengths of light are mixed at the inlet of the transmission line, and the mixture of light is separated into the plural wavelengths of light at the outlet. This requires an optical multiplexer and an optical demultiplexer, or wavelength filters. A type of demultiplexers currently used is arrayed weveguide grating (AWG). AWG uses a normal type of waveguide. With this construction, it is currently necessary to make the device as large as roughly several square centimeters to adequately decrease the loss of light.
Taking into account the above-described situation, research has been conducted on the miniaturization of demultiplexers by using a device composed of a photonic crystal as the multiplexer or demultiplexer, as disclosed in the Japanese Unexamined Patent Publication No. 2001-272555, which is referred to as “Prior Art Document 1” hereinafter. A brief description of the demultiplexer using a photonic crystal follows. When an appropriate defect is introduced in the photonic crystal, the defect creates an energy level, called the defect level, within the photonic bandgap. This state allows the light to exist only at a specific wavelength corresponding to the energy of the defect level within the wavelength range corresponding to the energies included in the photonic bandgap. There, when the defects in the crystal have a linear arrangement, the device functions as an optical waveguide, while it functions as an optical resonator when the defect or defects have a point-like form in the crystal.
When a ray of light including various wavelength components is propagated through the waveguide of a photonic crystal having an appropriate point defect located in proximity to the waveguide, only a specific component of light having a wavelength corresponding to the resonance frequency of the point defect is trapped by the point defect. Taking out the light will make the device a demultiplexer for a desired wavelength. In reverse, a ray of light having a wavelength corresponding to the resonance frequency may be introduced from the point defect into the photonic crystal and propagated through the waveguide with other components of light having different wavelengths. This makes the device a multiplexer for a desired wavelength.
For photonic crystals used as a multiplexer or demultiplexer, both two-dimensional and three-dimensional crystals may be used, each of which has its own features and advantages. The following description takes the two-dimensional crystal as an example, which is relatively easier to manufacture. In two-dimensional crystals, there is a significant difference in the refractive index in the direction perpendicular to its surface, or in the orthogonal direction, between the crystal body and the air so that it can confine light in the orthogonal direction.
Prior Art Document 1 discloses a study in which cylindrical holes of the same diameter are periodically arranged in a slab made of InGaAsP, a line of the cylindrical holes is filled to form an optical waveguide, and a defect is introduced by designing at least one of the cylindrical holes to have a diameter different from that of the other holes to create an optical resonator
In this structure, the lattice constant a is set at a value corresponding to the wavelength of light that should be propagated (in the above example, it is set at 1.55 μm, which is one of the wavelengths generally used in wavelength division multiplexing communications), the radius of the cylindrical hole formed at each lattice point is set at 0.29a except for one hole whose radius is set at 0.56a to create a point defect. With this configuration, a ray of light having a normalized frequency f=0.273 is radiated upwards and downwards from the point defect in the orthogonal direction of the slab. The Q-factor obtained thereby is about 500. It should be noted that a normalized frequency is a non-dimensional value obtained by multiplying the frequency of light by a/c, where c is the speed of light. The Q-factor indicates the quality of the resonator. The higher the Q-factor is, the higher the wavelength resolution. When the radius of one cylindrical hole is 0.56a and that of another hole is 0.58a, the normalized frequencies will be 0.2729 and 0.2769, respectively, so that two rays of light having different wavelengths are generated. The Q-factors for both holes are about 500.
As mentioned above, Prior Art Document 1 teaches that two-dimensional photonic crystals can be used as optical demultiplexers. These demultiplexers, however, need further improvements with regard to some points relating to Q-factor, as explained below. In the case of Prior Art Document 1, the Q-factor is about 500. With this value, the wavelength resolution of the above-described optical resonator is about 3 nm for the wavelength band of 1.55 μm, because the wavelength resolution of an optical resonator for wavelength λ is given by λ/Q. For a resonator to be applicable to high-density wavelength division multiplexing optical communications, however, the wavelength resolution must be about 0.8 nm or less, meaning that the Q-factor must be about 2000 or greater. One possible factor against the improvement in Q-factor in Prior Art Document 1 is an increase in the loss of energy of light in the orthogonal direction due to the introduction of the point defect.
The decrease in Q-factor may also result from asymmetry that may be introduced into a point defect. For example, as mentioned in Prior Art Document 1, a point defect may be designed to be asymmetrical in the direction orthogonal to the plane so that light is taken out only from one side of the two-dimensional plane. Otherwise, a point defect may be designed to be asymmetrical in the in-plane direction to take out a ray of polarized light. For example, when the point defect is circular, the light emitted from the defect is not polarized. However, it is often necessary to linearly polarize the light so as to couple it to an external optical system, or for some other purposes.
The present invention has been accomplished to solve such problems, and one of its objects is to provide an optical multiplexer/demultiplexer that can be smaller in size and higher in Q-factor or efficiency. Another object is to provide an optical multiplexer/demultiplexer that shows a high level of efficiency even when it has asymmetry introduced in the orthogonal direction or when polarized light needs to be obtained.